The present invention relates to an apparatus for controlling a rotation speed of a drive motor for a digital record reproducing apparatus.
In a digital signal recording and reproducing apparatus, a digital signal is frequently reproduced at a fixed clock rate. One example thereof is a digital disc reproducing apparatus.
In the disc reproducing apparatus in which a digital signal is reproduced by a pickup, a rotation speed of a disc drive motor is generally controlled in the following manners. One method is a constant angular velocity (CAV) system in which the rotation speed of the disc is maintained constant over an entire reproduction range. Another method is a constant linear velocity (CLV) system in which a relative velocity between the disc and the pickup is maintained constant.
In the CAV system, an information density in a unit track length is lower in an outer periphery than in an inner periphery, while in the CLV system it is constant over the entire range. Accordingly, the CLV system is more adapted to high density recording.
In the CLV system, however, it is necessary to increase the rotation speed of the disc drive motor as the pickup goes toward the inner periphery so that the relative speed between the disc and the pickup, that is, a linear speed is kept constant and the clock rate of the reproduced digital signal is kept constant.
A known control circuit for the CLV system is shown in FIG. 1, in which numeral 1 denotes a motor, numeral 2 denotes a disc on which a digital signal has been recorded, numeral 3 denotes a power supply, numeral 4 denotes a potentiometer, numeral 5 denotes a pickup such as a laser sensor, numeral 6 denotes a motor drive circuit and numeral 7 denotes a starting power supply.
The operation of the control circuit of FIG. 1 is explained below. When the starting voltage from the power supply 7 is applied to the drive circuit 6, the motor 1 starts to rotate up to a rotation speed determined by the starting voltage from the power supply 7. As the pickup 5 traces the disc 2 and goes toward the inner periphery, a division ratio of the potentiometer coupled to the pickup 5 varies. A voltage divided by the potentiometer 4 is added to the starting voltage from the power supply 7 in an adder and a sum voltage is supplied to the drive circuit 6. As a result, as the pickup 5 moves toward the inner periphery, the voltage supplied to the drive circuit 6 increases to increase the rotation speed of the motor so that the relative velocity between the disc 2 and the pickup 5, that is, the clock rate of the signal is kept constant. See, for example, Japanese Patent Application Laid-Open No. 54-12816.
In the prior art system described above, a high accuracy of operation is not attained because the position of the pickup 5 is detected by the potentiometer 4. In addition, if the initial setting is wrong, a correct rotation speed is not attained and hence a constant clock rate of the signal is not attained.